1. Field of the Invention
The present invention relates to a resin gear forming a power transmission device for various machines, such as printers, copying machines, video tape recorders and automotive parts, and a method for producing the same. The present invention also relates to an injection molded resin rotating body, such as a resin gear, a resin pulley or a resin sprocket, and a method for producing the same.
2. Description of the Prior Art
Conventional power transmission devices for machines, such as copying machines and automotive parts, use resin gears in order to reduce the price, weight and operation noises of parts. Typically, such a resin gear is formed so as to have a predetermined shape by injection molding. The shape of the resin gear is devised so that the resin gear has a desired precision of tooth profile and a desired strength in accordance with intended purpose.
FIGS. 8 and 9 show such a conventional injection molded resin gear 21. In the injection molded resin gear 21 shown in these figures, in order to decrease the amount of shrinkage in a process for cooling and solidifying a resin after injection molding to improve the precision of gear, a rim 23 having teeth 22 on its outer periphery is connected to a hub 25, into which a driving shaft 24 is to be fitted, by means of a thin-plate-like web 26 in radial directions. However, in such an injection molded resin gear 21 having the thin web 26, the rigidity of the web 26 deteriorates, so that there are some cases where a twist between the rim 23 and the hub 25 is given by external force acting thereon during power transmission to deteriorate the precision of transmission of rotation.
Therefore, in the conventional injection molded resin gear 21, circumferential ribs 27 are formed on both sides of the web 26 between the hub 25 and the rim 23 to be connected to the hub 25 by radial ribs 28 and to rim 23 by radial ribs 30, so that the rigidity of the web 26 is enhanced by the plurality of circumferential ribs 27 and radial ribs 28 and 30 (see, e.g., Japanese Patent Laid-Open No. 10-278124).
In such a conventional injection molded resin gear 21, as shown in FIGS. 8 and 9, there are some cases where injection molding gates 31 are arranged on a side of the web 26 on extension lines La which extend in radial directions from the radial ribs 30. Since a resin material injected into a cavity (not shown) from the gates 31, which are arranged on the extension lines La extending in radial directions from the radial ribs 30, is easy to flow along a space for forming the radial ribs 30 in the cavity (not shown), the resin material is late in flowing into a space for forming the web 26 in the cavity, so that the flow velocity of the resin material is not uniform in radial directions.
In addition, if the radial ribs 28 are arranged at positions at which resin materials injected from adjacent two of the gates 31 meet each other, after the resin materials injected from the adjacent two of the gates 31 meet each other, the resin materials are easy to flow in a space for forming the radial ribs 28 in the cavity.
As a result, when the resin material injected from the gates 31 flows in the cavity, the time for the resin material to reach outside and inside radial ends varies in accordance with the circumferential positions. Thus, the flow of the resin material is not uniform, so that it is difficult to uniformly apply an appropriate molding pressure on the rim 23 having the teeth 22 and on the hub 25 into which the driving shaft 24 is to be fitted. In addition, the cooling rate of the resin material is not uniform, and the amount of shrinkage during cooling and solidification varies in accordance with the circumferential positions, so that it is difficult to improve the precision of gear.
In particular, in the injection molded resin gear 21 formed by injection-molding a resin including a reinforced fiber, such as carbon fiber, since the radial flow velocity of the resin material injected from the gates 31 varies in accordance with the circumferential positions as described above, the orientation of the reinforced fiber is disturbed to cause a great difference in shrinkage during cooling and solidification of the resin material, and to cause a different in cooling solidification rate of the resin material, so that there is a problem in that the roundness (the precision of gear) of the toothed portion formed on the rim 23 deteriorates.